1. Field of the Invention
This invention relates to telephone signal transmission over subscriber loops and, more particularly, to range extension with gain for coil telephone loops.
2. Description of the Prior Art
On long subscriber loops, voice frequency signals are attenuated and direct current supervisory signals fall below the threshold value of central office detectors. These signals are aided by providing voice frequency gain and signaling range extension, respectively, in devices called range extenders with gain (REG). One such REG unit is disclosed in U.S. Pat. No. 4,282,407, issued Aug. 4, 1980, and assigned to the same assignee as the application herewith.
When a REG unit is used in a subscriber loop, the central office is directly connected with the telephone during the idle state ("on-hook"). When the telephone is detected to be "off-hook," an amplifier is switched into the path of the telephone signals and its gain is automatically set, based on the resistance of the loop. Such a REG unit, however, cannot be used with coin telephone loops because a coin control signal may be intepreted as an off-hook signal, causing the insertion of the amplifier into the coin loop. A transformer in the REG unit would then block direct current supervisory signals which control the collection or return of coins and which test for the presence of coins. Further, REG units do not aid coin tests, do not aid ground start, and do not transmit polarity reversals of battery from the central office, all required to operate the coin telephone.